Process for making photovoltaic devices and resultant product

ABSTRACT

A process and apparatus (70) for making a large area photovoltaic device (22) that is capable of generating low cost electrical power. The apparatus (70) for performing the process includes an enclosure (126) providing a controlled environment in which an oven (156) is located. At least one and preferably a plurality of deposition stations (74,76,78) provide heated vapors of semiconductor material within the oven (156) for continuous elevated temperature deposition of semiconductor material on a sheet substrate (24) including a glass sheet (26) conveyed within the oven. The sheet substrate (24) is conveyed on a roller conveyor (184) within the oven (156) and the semiconductor material whose main layer (82) is cadmium telluride is deposited on an upwardly facing surface (28) of the substrate by each deposition station from a location within the oven above the roller conveyor. A cooling station (86) rapidly cools the substrate (24) after deposition of the semiconductor material thereon to strengthen the glass sheet of the substrate.

This invention was made with Government support under SERI SubcontractNumber ZR-1-11059-1 with Solar Cells, Inc. awarded by the Department ofEnergy. The Government has certain rights in this invention.

This is a divisional of U.S. application Ser. No. 08/066,348 filed onMay 24, 1993 entitled APPARATUS FOR MAKING PHOTOVOLTIAC DEVICES, nowU.S. Pat. No. 5,372,646, which is a divisional of U.S. application Ser.No. 07/881,683 filed on May 12, 1992 entitled PROCESS AND APPARATUS FORMAKING PHOTOVOLTAIC DEVICES AND RESULTANT PRODUCT, now U.S. Pat. No.5,248,349 which issued on Sep. 28, 1993.

TECHNICAL FIELD

This invention relates to a process and apparatus for makingphotovoltaic devices and also relates to the resultant product forconverting light to electricity.

BACKGROUND ART

The photovoltaic effect was first observed in 1839 by Edmund Becquerelwhen he noted that a voltage appeared across two identical electrodes ina weak conducting solution that was subjected to light. Thisphotovoltaic effect was first studied in solids such as selenium in the1870's and by the 1880's, selenium photovoltaic cells were produced with1 to 2% efficiency in converting light to electricity.

Since the initial experimentation with photovoltaics over a century ago,much work has been conducted in developing semiconductors forphotovoltaic devices, i.e. solar cells. Much of the initial work wasdone with crystalline silicon which requires a relatively thick filmsuch as on the order of about 100 microns and also must be of very highquality in either a single-crystal form or very close to a singlecrystal in order to function effectively. The most common process formaking silicon cells is by the single-crystal cylinder process where asingle-crystal silicon seed crystal is touched to a molten silicon meltand then withdrawn to provide a raised meniscus of molten silicon withboth the seed crystal and the crucible holding the melt rotatedoppositely to enhance radial growth. Suitable doping will make the celleither an N-type or a P-type semiconductor and upon slicing into a waferof about 100 microns and formation of a junction will produce a solarcell or photovoltaic device. In addition, crystalline silicon can bemade by casting of an ingot but its solidification is not as easilycontrolled as with single-crystal cylinders such that the resultantproduct is a polycrystalline structure. Direct manufacturing ofcrystalline silicon ribbons has also been performed with good quality aswell as eliminating the necessity of cutting wafers to make photovoltaicdevices. Another approach referred to as melt spinning involves pouringmolten silicon onto a spinning disk so as to spread outwardly into anarrow mold with the desired shape and thickness. High rotational speedswith melt spinning increase the rate of formation but at thedeterioration of crystal quality.

More recent photovoltaic development has involved thin films which havea thickness less than 10 microns so as to be an order of magnitudethinner than thick film semiconductors. Such thin film semiconductorsinclude amorphous silicon, copper indium diselenide, gallium arsenide,copper sulfide and cadmium telluride. Amorphous silicon has been madeinto thin film semiconductors by plasma enhanced discharge, or glowdischarge, as disclosed by U.S. Pat. No. 5,016,562. Other processes usedto make thin film semiconductors include electrodeposition, screenprinting and close-spaced sublimation. The close-spaced sublimationprocess has been used with cadmium telluride and is performed byinserting a glass sheet substrate into a sealed chamber that is thenheated. The glass sheet substrate is supported at its periphery in avery close relationship, normally 2 to 3 mm, to a source material ofcadmium telluride. After the heating has proceeded to about 450°-500°C., the cadmium telluride begins to sublime very slowly into elementalcadmium and tellurium and, upon reaching a temperature of about650°-725° C., the sublimation is at a greater rate and the elementalcadmium and tellurium recombines at a significant rate as cadmiumtelluride on the downwardly facing surface of the peripherally supportedglass sheet substrate. The heating is subsequently terminated prior toopening of the chamber and removal of the substrate with the cadmiumtelluride deposited on the substrate. Thus, the deposition of thecadmium telluride is at a varying temperature that increases at thestart of the processing and decreases at the end of the processing.Furthermore, the largest area on which such close-spaced sublimation haspreviously been conducted is about 100 cm.² square. Increasing the sizeof the substrate can cause problems in maintaining planarity since theheated substrate which is supported at only its periphery will tend tosag at the center.

A more complete discussion of cadmium telluride processing is set forthin Chapter 11 of the book "Harnessing Solar Power-The PhotovoltaicsChallenge" by Ken Zweibel, published by Plenum Press of New York andLondon.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an improved process andapparatus for making photovoltaic devices capable of producing low costelectrical power which is achieved by the use of large area glass sheetsubstrates, i.e. over 1000 cm.², on which high quality semiconductormaterial is deposited at a relatively fast rate of deposition.

In carrying out the above object and other objects of the invention, theprocess for making a photovoltaic device in accordance with theinvention is performed by establishing a contained environment heated ina steady state during the processing to a temperature in a range aboveabout 650° C. and by introducing vapors of cadmium and tellurium intoone contained environment. A sheet substrate including a glass sheetheated to a temperature in the range of about 550° to 640° is conveyedwithin the contained environment for continuous elevated temperaturedeposition of a layer of cadmium telluride onto one surface of thesubstrate to function as a semiconductor for absorbing solar energy.

The substrate is oriented horizontally within the contained environmentwith the one surface of the substrate facing upwardly for the depositionof the cadmium telluride thereon and with the other surface of thesubstrate facing downwardly and being supported within the peripherythereof for horizontal conveyance. Most preferably, the substrate issupported and conveyed within the contained environment by horizontallyextending rolls of a roller conveyor during the deposition of the layerof cadmium telluride onto the one upwardly facing surface of thesubstrate in order to allow the deposition on large area substrates.

In the preferred practice of the process, another semiconductor materialis deposited onto the upwardly facing surface of the substrate as aseparate layer having an interface with the layer of cadmium telluridesuch that an electrical junction can be formed either upon the initialdeposition or by subsequent treatment. More specifically, anothersemiconductor material is deposited as another layer onto the onesurface of the substrate before the layer of cadmium telluride which isdeposited thereover and has an interface with the layer of cadmiumtelluride. This layer of semiconductor material deposited onto the onesurface of the substrate before the layer of cadmium telluride ispreferably cadmium sulfide. The additional layer of semiconductormaterial may also be deposited as another layer onto the one surface ofthe substrate after the layer of cadmium telluride so as to have aninterface with the layer of cadmium telluride on the opposite sidethereof from the substrate. As specifically disclosed, the process isperformed by depositing another semiconductor material as another layeronto the one surface of the substrate before the layer of cadmiumtelluride which is deposited thereover and has an interface with thelayer of cadmium telluride, and by also depositing a furthersemiconductor material as a further layer on the one surface of thesubstrate after the layer of cadmium telluride so as to have a furtherinterface with the layer of cadmium telluride.

In the preferred practice of the process, each layer of semiconductormaterial in addition to the layer of cadmium telluride is deposited byintroducing vapors into the contained environment, which is heated to atemperature in the range of about 650° C., for the deposition on the onesurface of the substrate during the conveyance thereof while heated tothe temperature range of about 550° to 640° C.

After the deposition of the layer of cadmium telluride, the processingis preferably performed by rapidly cooling the substrate at a rate thatprovides compressive stresses that strengthen the glass sheet. Morespecifically, the deposition of the layer of cadmium telluride is mostpreferably performed with the substrate heated to a temperature in therange of about 570° to 600° C., and thereafter the substrate is heatedto a temperature in the range of about 600° to 640° C. from which therapid cooling is performed to provide the compressive stresses tostrengthen the glass sheet.

A photovoltaic device constructed in accordance with the inventionincludes a sheet substrate including a planar glass sheet havingoppositely facing surfaces each of which has an area of at least 1000square centimeters. A thin-film layer of cadmium telluride is depositedon one of the surfaces of the substrate with a thickness in the range ofabout 1 to 5 microns and has crystals of a size in the range of about1/2 to 5 microns. The thin-film layer of cadmium telluride has a bond tothe one surface of the substrate by deposition thereon while the glasssheet is oriented horizontally and heated to a temperature in the rangeof about 550° to 640° C. within a contained environment that is heatedto a temperature in a range above about 650° C. and into which vapors ofcadmium and tellurium are introduced for deposition on the one surfacethereof as the layer of cadmium telluride on the surface thereof whichfaces upwardly while the other surface thereof faces downwardly and issupported within the periphery thereof for horizontal conveyance whilemaintaining the planarity of the glass sheet. This photovoltaic devicehas good crystal quality and good adherence to respectively enhance theefficiency and the effective lifetime of the relatively large areadevice so as to thereby achieve the object of the invention to providelow cost electrical power.

In the preferred construction of the photovoltaic device, a layer ofanother semiconductor material is deposited on the one surface of thesubstrate and has an interface with the layer of cadmium telluride. Thisadditional layer of another semiconductor material may be deposited onthe one surface of the substrate before the layer of cadmium tellurideto have an interface with the layer of cadmium telluride and, in suchcase, this additional layer of semiconductor material is preferablycadmium sulfide. The additional layer of another semiconductor may alsobe deposited on the one surface of the substrate after the layer ofcadmium telluride so as to have an interface with the layer of cadmiumtelluride. In the preferred construction disclosed, the photovoltaicdevice includes another layer of another semiconductor materialdeposited on the one surface of the substrate before the layer ofcadmium telluride and having an interface therewith, and thephotovoltaic device also includes a further layer of a furthersemiconductor material deposited on the one surface of the substrateafter the layer of cadmium telluride and having a further junction withthe layer of cadmium telluride.

Furthermore, the photovoltaic device also preferably includes a firstelectrically conductive film on the one surface of the substrate overwhich the initially deposited layer is deposited and further includes asecond electrically conductive film deposited on the one surface of thesubstrate over the finally deposited layer. These electricallyconductive films function as electrodes for the photovoltaic device.

In its preferred construction, the photovoltaic device has the glasssheet of the substrate heat strengthened with oppositely facing surfacesin compression and a central portion in tension, and the cadmiumtelluride has a bond that is cooled from tempering temperature andprovides adherence thereof to the one surface of the substrate.

The objects, features and advantages of the present invention arereadily apparent from the following detailed description of the bestmodes for carrying out the invention when taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic top plan view of a system for making photovoltaicdevices in accordance with the invention;

FIG. 2 is a plan view of a photovoltaic device constructed in accordancewith the invention;

FIG. 3 is an edge view of the photovoltaic device taken along thedirection of line 3--3 in FIG. 2 to illustrate its sheet construction;

FIG. 4 is a sectional view of the photovoltaic device taken in the samedirection as FIG. 3 but on an enlarged scale and partially broken awayto illustrate the construction of deposited semiconductor material andother materials on a glass sheet of the substrate;

FIG. 5 is a view that illustrates the manner in which the photovoltaicdevice is utilized to absorb solar energy and thereby provide electricalpower;

FIG. 6 is an elevational view taken in longitudinal section along thedirection of line 6--6 in FIG. 1 to illustrate apparatus of theinvention which includes a deposition zone having a plurality ofdeposition stations and also includes a cooling station locateddownstream from the deposition zone;

FIG. 7 is a cross-sectional view taken along the direction of line 7--7in FIG. 6 to further illustrate the construction of the deposition zone;

FIG. 8 is a partially broken away elevational view taken along thedirection of line 8--8 in FIG. 7 to further illustrate the constructionof the deposition station;

FIG. 9 is a somewhat schematic view illustrating another embodiment ofthe deposition station; and

FIG. 10 is a somewhat schematic view of still another embodiment of thedeposition station.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to FIG. 1 of the drawings, a system generally indicatedby 20 is constructed to manufacture photovoltaic devices 22 asillustrated in FIGS. 2 through 5. The type of photovoltaic device 22 orsolar cell manufactured by the apparatus includes a sheet substrate 24which includes a glass sheet 26 (FIG. 4) and has oppositely facingsurfaces 28 and 30 as is hereinafter more fully described. In order toproduce low cost electric power, the substrate 24 is of a large areawhich is greater than about 1000 square centimeters and the embodimentspecifically illustrated has a size of 60 centimeters by 120 centimetersso as to be approximately 2 feet by 4 feet. After deposition ofsemiconductor material 32 (FIG. 4) on the one surface 28 of thesubstrate 24 as well as after other processing hereinafter more fullydescribed, the completed photovoltaic device 22 has cells 34 which areillustrated as extending laterally between the opposite lateral sides 36of the substrate and are connected in series with each other as is alsohereinafter more fully described. Furthermore, it should be appreciatedthat the cells 34 could also extend longitudinally between the oppositeends 40 of the substrate and still function effectively. Electricalterminals 38 at the opposite ends 40 of the substrate provide forelectrical connection thereof as part of a photovoltaic field. Morespecifically as illustrated in FIG. 5, three of the photovoltaic devices22 are located end for end supported by a suitable frame 42 at asuitable angle by ground supports 44 so as to receive light from the sun46 and thereby produce electrical energy.

The processing of the system 20 shown in FIG. 1 begins with a sheetsubstrate 24 as shown in FIG. 4 having a glass sheet 26 that is 3/16inch (5 millimeters) thick with a film 48 of tin oxide applied byatmospheric pressure chemical vapor deposition 0.04 microns thick toimprove the optical quality when used for architectural purposes. Asilicon dioxide film 50 is applied by atmospheric pressure chemicalvapor deposition to a thickness of 0.02 microns over the tin oxide filmto provide a barrier. Another tin oxide film 52 that is 0.3 micronsthick and fluorine doped is applied over the silicon dioxide film 50 andfunctions as a reflective film in architectural usage with the fluorinedoping increasing the reflectivity. This second tin oxide film 50functions as an electrode for the photovoltaic device 22 as ishereinafter more fully described. Such a substrate 24 with the films 48,50 and 52 deposited on the glass sheet 26 is commercially available andis one starting product from which the photovoltaic device 22 can bemanufactured by the system 20 shown in FIG. 1.

With reference to FIG. 1, the system 20 includes a load station 54 onwhich the sheet substrate is loaded for the processing. After theloading, the substrate is transferred to a glass washing and dryingstation 56 of any commercially available type. A corner conveyor 58transfers the substrate from the washing and drying station 56 to alaser scribing station 60 that cuts through the tin oxide film 52 atscribe lines 62 (FIG. 4) to isolate the cells 34 from each other. Thescribed substrate is then transferred to another washing and dryingstation 64 to provide washing and drying prior to semiconductordeposition. Subsequently, the washed and dried substrate is transferredto a test/reject station 66 to make sure that the initial laser scribinghas isolated the cells.

With continuing reference to FIG. 1, the system 20 includes a suitableheater 68 for heating the substrate to a temperature in the range ofabout 550° to 640° C. in preparation for semiconductor deposition.Thereafter, the substrate is transferred to apparatus 70 constructed inaccordance with the present invention and including a deposition zone 72which is disclosed as having three deposition stations 74, 76 and 78 fordepositing layers of semiconductor material. More specifically, thefirst deposition station 74 deposits a cadmium sulfide layer 80 (FIG. 4)that is 0.05 microns thick and acts as an N-type semiconductor. Thedeposition station 76 shown in FIG. 1 deposits a cadmium telluride layer82 which is 1.6 microns thick and acts as an I-type semiconductor.Thereafter, the deposition station 78 deposits another semiconductorlayer 84 (FIG. 4) which as disclosed is 0.1 microns thick and is zinctelluride that acts as a P-type semiconductor. The semiconductor layers80 and 82 have an interface 81 for providing one junction of the N-Itype, while the semiconductor layers 82 and 84 have an interface 83 forproviding another junction of the I-P type. These interfaces 81 and 83normally are not abrupt on an atomic scale, but rather extend over anumber of atomic layers in a transition region.

Apparatus 70 of the present invention is also illustrated in FIG. 1 asincluding a cooling station 86 that provides rapid cooling of the glasssheet substrate with the semiconductor material deposited thereon so asto strengthen the glass sheet as is hereinafter more fully described.

A corner conveyor 88 shown in FIG. 1 receives the substrate from thecooling station 86 and may also provide additional cooling thereof priorto transfer to a pin hole repair station 90. A suitable scanner of thepin hole repair station 90 scans the substrate to detect any pin holesin the deposited semiconductor layers by passing the substrate over abacklighted zone and then transferring the information to a computercontrolled multiple head delivery system that fills the void with asuitable viscous nonconductive material. After such repair, thesubstrate is transferred to a second laser scribing station 92 that cutsscribes 94 (FIG. 4) through the semiconductor layers 80, 82 and 84between the opposite lateral sides of the substrate at spaced locationsfrom the scribes 62 in the tin oxide layer 52 which acts as theelectrode. After the semiconductor scribing at station 92, the substrateis received by a corner conveyor 96 which also includes a suitableblower and vacuum for removing semiconductor material that is loosenedby the scribing.

A sputtering station 98 receives the substrate from the corner conveyor96 shown in FIG. 1 and deposits a nickel layer 100 (FIG. 4) over thesemiconductor layers and on the sides and bottom surface of the scribelines 94. This nickel sputtering is preferably performed by directcurrent magnetron sputtering and need only be about 100 angstroms thickto provide a stable contact for a subsequent deposition. Thereafter, thesubstrate is transferred to a sputtering station 102 that deposits analuminum layer 104 which is 0.3 microns thick over the nickel layer 100to act as an electrode on the opposite side of the semiconductor layersas the tin oxide film 52 which acts as the other electrode. The aluminumlayer 104 is deposited by in-line multiple cathode, direct currentmagnitron sputtering. Thereafter the substrate is received by anothersputtering station 106 that applies another nickel layer 108 over theelectrode aluminum layer 104 to prevent oxidation of the aluminum layer.

A third laser scribing station 110 shown in FIG. 1 receives thesubstrate from the sputtering station 106 and then cuts scribe lines 112(FIG. 4) through the electrode aluminum layer 104 and its adjacentnickel layers 100 and 108 as well as through the semiconductor layers tocomplete the isolation of the cells 34 between the opposite lateralsides of the substrate. Upon exiting the scribing station, a blower 114removes any loose particle from the substrate prior to transferring to amodule station 116 that tests the resultant photovoltaic deice under apredetermined illumination and the electrical output is measured forcomparison with a standard to determine whether the product issatisfactory. The satisfactory substrates are then transferred to anassembly station 118 where bus bars are ultrasonically welded to theends of each substrate and wire leads are soldered to the bus bars foruse in connecting the photovoltaic device within an array. Thereafter,the photovoltaic devices 22 are transferred to an encapsulation station120 where a suitable encapsulant layer 122 (FIG. 4) is applied and curedwithin an ultraviolet light chamber prior to transfer to an unloadstation 124. Subsequently, the completed photovoltaic devices 22 areassembled as previously described in connection with FIG. 5 into panelsfor constructing a photovoltaic array that generates electrical power.

It should be appreciated that the system 20 which with the apparatus 70of this invention is used can be constructed with other stations thanthe ones illustrated. For example, rather than the laser scribingstations 60, 92 and 110 for defining the cells 34, it is possible to usephotolithographic patterning to provide the cells.

With reference to FIGS. 6 and 7, the apparatus 70 for making aphotovoltaic device in accordance with the present invention includes anenclosure 126 that extends between the heater 68 and the corner conveyor88 previously described. This enclosure 126 as best illustrated in FIG.7 include a lower wall 128, an upper wall 130, and side walls 132 aswell as lower and upper seals 134 and 136 that seal between the walls soas to provide an enclosed interior capable of containing a controlledenvironment. Suitable fasteners or clamps can be utilized to maintainthe sealed condition of the enclosure 126.

As illustrated in FIG. 6, the right upstream end of the enclosure 126includes an entry valve 138 whose actuator 140 moves a valve element 142to open the enclosure to receive a heated substrate 24 from the heater68. Thereafter, the actuator 140 closes the valve element 142 to sealthe enclosure. After the semiconductor deposition at the depositionstations 74, 76 and 78 of the deposition zone 72 as is hereinafter morefully described, another valve 144 at the downstream left end of thedeposition zone is operated so that its actuator 146 opens a valveelement 148 in order to allow the substrate 24 with the semiconductormaterials coated thereon to pass to the cooling station 86. As thistransfer takes place, a further valve 150 at the downstream end of thecooling station 86 is closed with its actuator 152 positioning a valveelement 154 thereof in a closed position with respect to the downstreamend of the enclosure 126. Valve 144 is closed after the transfer of thesubstrate 24 to the cooling station 86 and the valve 150 is thereafteropened to allow the cooled substrate 24 to be transferred from thecooling station 86 to the conveyor 88 for continued processing aspreviously described.

As best illustrated in FIGS. 6, 7 and 8, the deposition zone 72 of theapparatus includes an oven 156 located within the enclosure 126 andhaving a housing 158 that defines a heated chamber 160 that iscommunicated with the interior of the enclosure 126 such that thecontrolled environment therein is also within the heated chamber. Thisoven housing 158 includes lower and upper cooling plates 162 and 164which have respective coolant passages 166 and 168 through which asuitable coolant flows. Housing 158 also includes lower and upperinsulator walls 170 and 172 made from suitable insulation as well asincluding lower and upper side insulator walls 174 and 176 also made ofsuitable insulation. These insulator walls 170, 172, 174 and 176cooperatively define the heated chamber 160 in which the semiconductordeposition takes place as is hereinafter more fully described. Thissemiconductor deposition is performed at elevated temperature providedby electrical heater elements 178 mounted by insulators 180 on the lowerinsulator wall 170 as shown in FIG. 8 and by suitable electricresistance heater elements 182 embedded within the upper insulator wall172.

With combined reference to FIGS. 6 and 7, the apparatus 70 also includesa roller conveyor 184 having horizontal rolls 186 spaced from each otherwithin the heated chamber 160 to support and convey the heated substrate24 which, as previously described includes a glass sheet. Each roll 186as best illustrated in FIG. 7 has at least one end 188, and preferablyboth of its ends, extending outwardly from the heated chamber 160 of theoven 156 through the housing 158 of the oven. More specifically, theroll ends 188 as shown in FIG. 8 extend outwardly through holescooperatively provided by semicircular openings 190 and 192 in the lowerand upper side insulator walls 174 and 176 at their engaged interface194. The roll ends 188 are driven by a roll drive mechanism 196 of theroller conveyor 184. This roll drive mechanism 196 as shown in FIG. 7 islocated within the interior of the enclosure 126 externally of the oven156 to rotatively drive the roll ends as is hereinafter more fullydescribed in order to convey the substrate 24 during the semiconductordeposition.

The deposition zone 72 of the apparatus 70 as illustrated in FIG. 6 andas previously discussed includes at least one deposition station andpreferably three of the deposition stations 74, 76 and 78 within theoven 156 for supplying heated vapor that is deposited as a layer ofsemiconductor material onto the upwardly facing surface of the substrate24 during conveyance thereof on the roller conveyor 184. Morespecifically, the one deposition station 74 provides heated vapors ofcadmium sulfide that are deposited as the cadmium sulfide layer 80 (FIG.4), while the deposition station 76 shown in FIG. 6 provides heatedvapors that are deposited as the cadmium telluride layer 82 (FIG. 4)which has the interface 81 with the cadmium sulfide layer 80.Furthermore, the deposition station 78 shown in FIG. 6 provides heatedvapors that are deposited as a further semiconductor layer such as thezinc telluride layer 84 that has the interface 83 with the cadmiumtelluride layer 82.

With combined reference to FIGS. 7 and 8, the one deposition station 76which is also illustrative of the deposition station 74 and 78 isdisclosed as including a source material holder 198 located within theoven 156 above the roller conveyor 184 to receive the source material200 which in this instance is the cadmium telluride that is the mainsemiconductor material being deposited by the apparatus. This sourcematerial sublimes due to the heated condition of the oven chamber 160 toprovide elemental cadmium and tellurium vapors that are deposited on theconveyed substrate 24 supported by the roller conveyor 184. Morespecifically, the source material holder 198 of the deposition station176 includes at least one holder trough 202 that opens upwardly toreceive the source material 200 and, preferably, there are a pluralityof such holder troughs as is hereinafter more fully described. Thedeposition station also includes a deflector 204 located above thesource material trough 202 and having a downwardly opening shape. Asillustrated, there are a plurality of the holder troughs 202 which haveelongated shapes and open upwardly extending parallel to the conveyorrolls with their opposite ends mounted by suitable supports 206 on theupper side insulator walls 176 within the oven chamber 160. Likewise,the deflectors 204 also have elongated shapes extending parallel to theconveyor rolls 186 and have their opposite ends mounted by the supports206 on the upper side insulator walls 176. Both the troughs 202 and thedeflectors 204 are preferably made from quartz so as to be capable ofwithstanding the elevated temperature to which the oven is heated duringthe semiconductor deposition. Furthermore, the deflectors 202 not onlydirect the heated vapors downwardly toward the substrate 24 for thedeposition but also provide a shield that prevents material from fallingdownwardly from above and destroying the semiconductor quality beingdeposited. Also, as shown in FIG. 6, a pair of baffles 207 defines aslit that allows conveyance of the substrate 24 from deposition station74 to the deposition station 76 but the baffles restrict the flow ofheated vapors between these stations. Another pair of like baffles 207provides the same function between deposition stations 76 and 78.

With reference to FIG. 9, another embodiment of the deposition station76a includes a heated vapor supply 208 whose temperature can becontrolled without affecting the temperature of the substrate 24 withinthe oven 156. The heated vapor supply includes a supply conduit 210 forproviding a means for feeding heated vapor for deposition on thesubstrate 24 conveyed by the conveyor 184. More specifically, thisembodiment is illustrated as including a heater 212 in which the sourcematerial 200 such as cadmium telluride is heated to provide the heatedvapors of cadmium and tellurium supplied through the conduit 210 to theoven 156. This heater 212 is shown as being located externally of boththe oven 156 and externally of the enclosure 126 with the conduit 210extending into the enclosure and into the oven to supply the heatedvapors for deposition. However, it should be appreciated that the heater212 also could be located within the enclosure 126 externally of theoven 156 or within the oven heated chamber 160 without affecting thetemperature of the substrate 24, such as by the use of insulation and/orremoteness in location from the substrate. Furthermore, source 214 of acarrier gas such as nitrogen is fed through a control valve 216 toassist in transferring the heated vapors from the heater 212 to the oven156. Another advantage of the externally located source material heater212 is that it is easily controlled independently of the temperaturewithin the oven 156 in providing heating of the source material asnecessary for the deposition.

With reference to FIG. 10, a further embodiment of the depositionstation 76b within the oven 156 is illustrated as also including aheated vapor supply 208 which includes a pair of the source materialheaters 212' for respectively heating elemental cadmium and telluriumindependently of each other. These heaters provide heated vapors throughthe conduit 210 to the interior of the oven 156 preferably by the use ofa carrier gas such as nitrogen supplied from the source 214 through thecontrol valves 216' and 216" respectively associated with the conduitbranches 210' and 210".

As illustrated in FIG. 6, the drive mechanism 196 includes a continuousdrive loop 218 having a driving reach 220 and a return reach 222. In thepreferred construction illustrated in FIG. 7, there are a pair of thedrive loops 218 that respectively support and rotatively drive theconveyor roll ends 188 at the opposite lateral sides of the oven. Thisdrive loop 218 is preferably embodied by a drive chain which, as shownin FIG. 6, is received at its upstream end by a drive sprocket 224 andat its downstream end by another sprocket 226. Upper supports 228 on theenclosure side walls 132 as shown in FIG. 7 support the driving reaches220 of the pair of drive loops 218 while lower supports 230 slidablysupport the lower return reaches 222 of the drive loops. A suitableelectric motor driven drive shaft extending into the enclosure 126through a seal rotatively drives the drive sprockets 220 that receivethe drive chains in a counterclockwise direction to move the upperdriving reaches 220 toward the right and thereby frictionally drive theconveyor rolls 186 counterclockwise to conveyor the substrates 24 towardthe left. Positioners 232 shown in FIG. 7 may include suitable rollerswhich engage the roll ends 188 to position the conveyor rolls 186 alongthe length of the apparatus through both the deposition zone 72 and thecooling station 86.

Furnaces for heating glass sheets within the ambient, as opposed to acontrolled environment within an enclosure in accordance with thepresent invention, are disclosed by U.S. Pat. Nos. 3,934,970; 3,947,242;and 3,994,711. These furnaces have roller conveyors whose rolls arefrictionally driven by continuous drive loops, i.e. chains, in the samemanner as the roller conveyor of this invention.

With reference to FIG. 6, the cooling station 86 includes lower andupper blastheads 234 and 236 respectively located below and above theroller conveyor 184 and having nozzles for supplying quenching gas suchas nitrogen that rapidly cools the substrate 24 with the semiconductormaterial deposit thereon to thereby strengthen the glass sheet of thesubstrate. More specifically, this rapid cooling places the oppositelyfacing surfaces of the glass sheet in compression and its centralportion between the surfaces in tension.

The process by which the photovoltaic device is made within theapparatus 70 described above begins by establishing a containedenvironment within the oven 156 by drawing a vacuum or establishinganother controlled environment within the enclosure 126. This controlledor contained environment may include a suitable inert gas or an inertgas along with oxygen so long as there is no variable or variables thatdisrupt the controlled semiconductor deposition, such as variable watervapor in the atmosphere. It is also possible, as previously mentioned,to have a vacuum that constitutes the controlled environment containedwithin the enclosure 126 and hence also within the oven 156. The extentof the vacuum may be varied to provide best results. For example, it hasbeen found that a vacuum of 5 torr is better than a vacuum of 1 torr inthat there is a shorter mean free path for the heated semiconductormaterial vapors that are supplied during the processing as previouslydescribed and that there is less vapor travel and deposition on theopposite side of the substrate from the intended side on which thedeposition is to take place. On the other hand, the deposition rate ishigher at a lower pressure and, in addition, the uniformity ofdeposition depends on both pressure and temperature. Furthermore, thecontained environment is heated to a temperature range above about 650°C. so the cadmium telluride does not deposit onto the oven walls, andthe material holder 198 most preferably is heated to about 700° C. tosublime the cadmium telluride source material 200 at a sufficiently fastrate for rapid deposition.

The processing proceeds by introduction of heated vapors which for themain semiconductor material described above are of cadmium andtellurium. Conveyance of the sheet substrate 24 including the glasssheet 26 heated to a temperature in the range of about 550° to 640° C.within this contained environment provides continuous elevatedtemperature deposition of a layer of cadmium telluride onto the onesurface 28 of the substrate as previously described so as to function asa semiconductor for absorbing solar energy. This cadmium telluride layer82 as shown in FIG. 4 has an interface 81 with the cadmium sulfide layer80 to provide an N-I junction adjacent the glass sheet side of thecadmium telluride layer. Likewise, the interface 83 of the cadmiumtelluride layer 82 with the zinc telluride layer 84 or another P-typesemiconductor provides a P-I junction such that the resultantphotovoltaic device is of the N-I-P type.

As previously described, the processing is preferably performed with thesubstrate 24 oriented horizontally within the contained environment withthe one surface 28 of the substrate facing upwardly for the depositionof the cadmium telluride thereon and with the other surface 30 of thesubstrate facing downwardly and being supported within the peripherythereof for horizontal conveyance. This support of the substrate ispreferably by the horizontally extending rolls 186 of the rollerconveyor 184 during the deposition of the layer of cadmium tellurideonto the upwardly facing surface 28 of the substrate, and such supportallows relatively large glass sheet substrates to be continuouslysubjected to the semiconductor deposition while maintaining planaritydespite the softness and tendency of the glass sheet to sag at itsheated condition.

The processing proceeds as previously discussed in connection with FIG.6 through the three deposition stations 74, 76 and 78 to deposit each ofthe semiconductor layers 80, 82 and 84 with the layers 80 and 82 havingan interface 81 with each other and with the layers 82 and 84 having aninterface 83 with each other. As discussed above, best results areachieved when the cadmium sulfide layer is deposited on the substratesurface 28 before the cadmium telluride layer 82 and when another P-typesemiconductor layer 84 is deposited after the cadmium telluride layer.

After the deposition of the semiconductor materials as described above,the heated substrate is rapidly cooled within the cooling station 86 ata rate that provides compressive stresses that strengthen the glasssheet. More specifically, this processing is preferably performed byhaving the deposition of the layer of cadmium telluride deposited withthe substrate 24 heated to a temperature in the range of about 570° to600° C. and thereafter heating the substrate to a temperature in therange of about 600° to 640° C. from which the rapid cooling is performedto provide the compressive stresses that strengthen the glass sheet.Such processing reduces the time during which the glass sheet is in anelevated temperature so as to tend to sag while still providing asufficiently heated condition prior to the cooling so as to facilitatethe compressive stress build-up that strengthens the glass sheet.

The resultant photovoltaic device 22 made by the apparatus andprocessing described above has a thin-film layer 82 of the cadmiumtelluride deposited on the one surface 28 of the substrate 24 with athickness that is effective within the range of about 1 to 5 microns andwhich has crystals of a size in the range of about 1/2 to 5 microns.This thin-film layer 82 of the cadmium telluride has an enhanced bond tothe one surface of the substrate by virtue of the deposition thereonwhile the glass sheet is heated to the temperature in the range of about550° to 640° C. within the contained environment that is heated to atemperature above about 650° C. as previously described and into whichvapors of cadmium and tellurium are introduced. This introductionprovides the deposition on the one substrate surface 28 as the layer 80of cadmium telluride.

Furthermore, the photovoltaic device 22 has the construction previouslydescribed in connection with FIG. 4 with respect to the othersemiconductor layers and films which are deposited thereon to providethe electrodes and the cells which are separated from each other butconnected in series through the semiconductor layers. It should beemphasized that the heat strengthening of the glass sheet 26 of thesubstrate by cooling from tempering temperature provides enhancedadherence of the cadmium telluride to the one surface 28 of thesubstrate.

As is apparent from the above description, the process and apparatusdescribed produces a photovoltaic device that is capable of providinglow cost electrical power generation.

While the best modes for carrying out the invention have been describedin detail, other processes, apparatus and photovoltaic devices accordingto the invention are possible as defined by the following claims.

What is claimed is:
 1. A process for making a photovoltaic device,comprising:establishing a contained environment heated in a steady stateduring the processing; introducing vapors of cadmium and tellurium intothe contained environment; and conveying a heated sheet substrateincluding a planar glass sheet within the contained environment forcontinuous elevated temperature deposition of a layer of cadmiumtelluride onto one surface of the substrate to function as asemiconductor for absorbing solar energy, the substrate being orientedhorizontally within the contained environment with the one surface ofthe substrate facing upwardly for the deposition of the cadmiumtelluride thereon and with the other surface of the substrate facingdownwardly and being supported within the periphery thereof forhorizontal conveyance while maintaining the planarity of the glasssheet.
 2. A process for making a photovoltaic device as in claim 1wherein the substrate is supported and conveyed within the containedenvironment by horizontally extending rolls of a roller conveyor duringthe deposition of the layer of cadmium telluride onto the one upwardlyfacing surface of the substrate.
 3. A process for making a photovoltaicdevice as in claim 1 wherein another semiconductor material is depositedonto the upwardly facing surface of the substrate as a separate layerhaving an interface with the layer of cadmium telluride.
 4. A processfor making a photovoltaic device as in claim 1 wherein anothersemiconductor material is deposited as another layer onto the onesurface of the substrate before the layer of cadmium telluride which isdeposited thereover and has an interface with the layer of cadmiumtelluride.
 5. A process for making a photovoltaic device as in claim 4wherein the layer of semiconductor material deposited onto the onesurface of the substrate before the layer of cadmium telluride iscadmium sulfide.
 6. A process for making a photovoltaic device as inclaim 1 wherein another semiconductor material is deposited as anotherlayer onto the one surface of the substrate after the layer of cadmiumtelluride and has an interface with the layer of cadmium telluride.
 7. Aprocess for making a photovoltaic device as in claim 1 wherein anothersemiconductor material is deposited as another layer onto the onesurface of the substrate before the layer of cadmium telluride which isdeposited thereover and has an interface with the layer of cadmiumtelluride, and wherein a further semiconductor material is deposited asa further layer on the one surface of the substrate after the layer ofcadmium telluride and has a further interface with the layer of cadmiumtelluride.
 8. A process for making a photovoltaic device as in any oneof claims 3 through 7 wherein each layer of semiconductor material inaddition to the layer of cadmium telluride is deposited by introducingvapors into the contained environment for the deposition on the onesurface of the substrate during the conveyance thereof.
 9. A process formaking a photovoltaic device as in claim 1 wherein after the depositionof the layer of cadmium telluride the substrate is rapidly cooled at arate that provides compressive stresses that strengthen the glass sheet.10. A process for making a photovoltaic device as in claim 9 wherein thedeposition of the layer of cadmium telluride is performed with thesubstrate heated to a temperature in the range of about 570° to 600° C.,and thereafter the substrate being heated to a temperature in the rangeof about 600° to 640° C. from which rapid cooling is performed toprovide the compressive stresses that strengthen the glass sheet.
 11. Aprocess for making a photovoltaic device comprising:establishing acontained environment heated in a steady state during the processing;introducing vapors of cadmium and tellurium into the containedenvironment; and conveying a heated sheet substrate including a planarglass sheet within the contained environment horizontally onhorizontally extending rolls for continuous elevated temperaturedeposition of a layer of cadmium telluride onto one upwardly facingsurface of the substrate to function as a semiconductor for absorbingsolar energy, the substrate being oriented horizontally within thecontained environment by the horizontally extending rolls to provide theone upwardly facing surface of the substrate on which the deposition ofthe cadmium telluride takes place and with the other surface of thesubstrate facing downwardly and being supported by the rolls within theperiphery of the substrate for horizontal conveyance while maintainingthe planarity of the glass sheet.
 12. A process for making aphotovoltaic device comprising:establishing a contained environmentheated in a steady state during the processing; introducing vapors ofcadmium and tellurium into the contained environment; conveying a heatedsheet substrate including a planar glass sheet within the containedenvironment horizontally on horizontally extending rolls for continuouselevated temperature deposition of a layer of cadmium telluride onto oneupwardly facing surface of the substrate to function as a semiconductorfor absorbing solar energy, the horizontally extending conveyor rollssupporting the substrate within the periphery thereof for horizontalconveyance while maintaining the planarity of the glass sheet; andthereafter rapidly cooling the substrate with the layer of cadmiumtelluride deposited thereon to provide compressive stresses thatstrengthen the glass sheet.
 13. A photovoltaic device comprising: asheet substrate which includes a planar glass sheet and has oppositelyfacing surfaces each of which has an area of at least 1000 cm.² ; athin-film layer of cadmium telluride deposited on one of the surfaces ofthe substrate with a thickness in the range of about 1 to 5 microns andhaving crystals of a size in the range of about 1/2 to 5 microns; andthe thin-film layer of cadmium telluride having a bond to the onesurface of the substrate by deposition thereon while the glass sheet isoriented horizontally and heated within a contained environment that isheated and into which vapors of cadmium and tellurium are introduced fordeposition as the layer of cadmium telluride on the one surface thereofwhich faces upwardly while the other surface thereof faces downwardlyand is supported within the periphery thereof for horizontal conveyancewhile maintaining the planarity of the glass sheet.
 14. A photovoltaicdevice as in claim 13 further including a layer of another semiconductormaterial deposited on the one surface of the substrate and having aninterface with the layer of cadmium telluride.
 15. A photovoltaic deviceas in claim 13 further including another layer of another semiconductormaterial deposited on the one surface of the substrate before the layerof cadmium telluride and having an interface with the layer of cadmiumtelluride.
 16. A photovoltaic device as in claim 15 wherein the layer ofsemiconductor material deposited on the one surface of the substratebefore the layer of cadmium telluride is cadmium sulfide.
 17. Aphotovoltaic device as in claim 13 further including another layer ofanother semiconductor material deposited on the one surface of thesubstrate after the layer of cadmium telluride and having an interfacewith the layer of cadmium telluride.
 18. A photovoltaic device as inclaim 13 further including another layer of another semiconductormaterial deposited on the one surface of the substrate before the layerof cadmium telluride and having an interface therewith, and a furtherlayer of a further semiconductor material deposited on the one surfaceof the substrate after the layer of cadmium telluride and having afurther interface with the layer of cadmium telluride.
 19. Aphotovoltaic device as any one of claims 14 through 18 further includinga first electrically conductive film on the one surface of the substrateover which the initially deposited layer is deposited, and a secondelectrically conductive film deposited on the one surface of thesubstrate over the finally deposited layer.
 20. A photovoltaic device asin claim 19 wherein the glass sheet of the substrate is heatstrengthened and has oppositely facing surfaces in compression and acentral portion that is in tension; and the cadmium telluride having abond that is cooled from tempering temperature and provides adherencethereof to the one surface of the substrate.
 21. A photovoltaic devicecomprising: a substrate which has oppositely facing surfaces each ofwhich has an area of at least 1000 cm.² ; the substrate including aplanar glass sheet that is heat strengthened and has oppositely facingsurfaces in compression and a central portion that is in tension; athin-film layer of cadmium telluride deposited on one of the surfaces ofthe substrate with a thickness in the range of about 1 to 5 microns andhaving crystals of a size in the range of about 1/2 to 5 microns; andthe thin-film layer of cadmium telluride having a bond to the onesurface of the substrate by deposition thereon while the glass sheet isoriented horizontally and heated within a contained environment that isheated and into which vapors of cadmium and tellurium are introduced fordeposition on the one surface thereof as the layer of cadmium tellurideon the one surface thereof which faces upwardly while the other surfacethereof faces downwardly and is supported within the periphery thereoffor horizontal conveyance while maintaining the planarity of the glasssheet.
 22. A photovoltaic device as in claim 21 wherein the substrateincludes a first electrically conductive film on the one surfacethereof, another layer of another semiconductor material deposited onthe one surface of the substrate and having an interface with the layerof cadmium telluride, and a second electrically conductive filmdeposited on the one surface of the substrate over the finally depositedlayer.
 23. A photovoltaic device as in claim 21 wherein the substrateincludes a first electrically conductive film on the one surfacethereof, another layer of another semiconductor material depositedbetween and having interfaces with both the first electrical film andthe layer of cadmium telluride, a further layer of a furthersemiconductor material deposited over the layer of cadmium telluride andhaving an interface therewith, and a second electrically conductive filmdeposited over the further layer of the further semiconductor material.